The present invention relates to alarm devices for vapour generators. The device is particularly adapted to detect when the liquid level in the generator moves past a predetermined level and then produce a signal.
The invention will be described with particular reference to devices for detecting and signalling water level changes in steam boilers. However it will be appreciated that the device could be used equally well in systems where the working fluid is other than water.
The device finds particular use in the provision of alarm devices for giving a warning of high and/or low level conditions in steam boilers operating at high pressure, e.g. at 2500 p.s.i., though it is also useful for low pressure boilers as well.
The provision of high and low level alarms for high pressure water boilers presents a number of problems. Float or displacement alarms, which may be sufficient (though still having some defects) for low pressure boilers, and which are used to actuate steam whistles or electrical warning devices are not satisfactory in high pressure uses. The float may have to be too substantial and the float chamber too massive to be practical. Steam density increases with pressure but that of the water decreases so that the differential force available to operate the float diminishes. Steam whistles suffer from the needle valve seat becoming scored as a result of the high pressure drop, causing a permanent leak-off. Magnetic relay systems have to be safeguarded against loss of magnetism due to overheating and also the mechanism becoming fouled, caused by the magnet's attraction for any iron oxide or iron particles in the boiler water.
Instrument type sensing devices depending upon the differential pressure between a static head and that in the boiler drum are equally affected by the small difference between the densities of high pressure steam and water. In the case of a boiler operating at about 2500 lb. per sq. in., for instance, the 1 inch change of water level to which an alarm would be expected to respond represents a change in head equivalent of only 0.015 lb. per sq. in. A very delicate instrument would be required to record a pressure change of this order.
Thermostatic water level alarms have been proposed, e.g. in British Patent Specification No. 608867, but have not been satisfactory. Such arrangements use inclined metal expansion tubes of various shapes, connected to the boiler drum in the same manner as the gauge glass, so that any movement of the water level is transmitted to the tube. When the boiler level rises, water rising in the tube cools it, causing contraction. When the level falls, the tube contains more steam and expanded with a consequent temperature rise. This expansion and contraction is multiplied by leverage and applied to a micro switch or similar device which operates an electric bell or siren, and indicator lights.
The main defects in systems such as these is inadequate temperature difference between steam and water in the tube. Continuous condensation in the upper part of the tube drains down and raises the water temperature close to that of the steam. To sound the alarm at high water level, therefore, it is expected to contract a tube heated to steam temperature by using very hot condensate as the cooling medium. This condition would become worse at the higher boiler pressure where greater heat dissipation due to higher temperature coupled with the reduced latent heat would promote an even greater rate of condensation.
A secondary problem to be solved arises from the fact that steam temperature changes with steam pressure, so a pressure change would affect the mean temperature of the expansion tube in the same way as a water level change. Therefore, unless some form of compensation was incorporated in the mechanism, the alarm would need to be readjusted whenever the boiler pressure varied outside the narrow set limits.